[PATCH v2 15/18] arm64: arch_timer: Enable CNTVCT_EL0 trap if workaround is enabled

[Marc Zyngier]

Userspace being allowed to use read CNTVCT_EL0 anytime (and not
only in the VDSO), we need to enable trapping whenever a cntvct
workaround is enabled on a given CPU.

Signed-off-by: Marc Zyngier <marc.zyng...@arm.com>
---
 drivers/clocksource/arm_arch_timer.c | 42 +++++++++++++++++++++++++-----------
 1 file changed, 30 insertions(+), 12 deletions(-)

diff --git a/drivers/clocksource/arm_arch_timer.c 
b/drivers/clocksource/arm_arch_timer.c
index 70eb42bd6784..eccff0fedf2c 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -83,6 +83,7 @@ static enum ppi_nr arch_timer_uses_ppi = VIRT_PPI;
 static bool arch_timer_c3stop;
 static bool arch_timer_mem_use_virtual;
 static bool arch_counter_suspend_stop;
+static bool vdso_default = true;
 
 static bool evtstrm_enable = IS_ENABLED(CONFIG_ARM_ARCH_TIMER_EVTSTREAM);
 
@@ -390,6 +391,17 @@ void arch_timer_enable_workaround(const struct 
arch_timer_erratum_workaround *wa
        }
 
        static_branch_enable(&arch_timer_read_ool_enabled);
+
+       /*
+        * Don't use the vdso fastpath if errata require using the
+        * out-of-line counter accessor. We may change our mind pretty
+        * late in the game (with a per-CPU erratum, for example), so
+        * change both the default value and the vdso itself.
+        */
+       if (wa->read_cntvct_el0) {
+               clocksource_counter.archdata.vdso_direct = false;
+               vdso_default = false;
+       }
 }
 
 static void arch_timer_check_ool_workaround(enum arch_timer_erratum_match_type 
type,
@@ -450,11 +462,19 @@ static void arch_timer_check_ool_workaround(enum 
arch_timer_erratum_match_type t
        __val;                                                          \
 })
 
+static bool arch_timer_this_cpu_has_cntvct_wa(void)
+{
+       const struct arch_timer_erratum_workaround *wa;
+
+       wa = __this_cpu_read(timer_unstable_counter_workaround);
+       return wa && wa->read_cntvct_el0;
+}
 #else
 #define arch_timer_check_ool_workaround(t,a)           do { } while(0)
 #define erratum_set_next_event_tval_virt(...)          ({BUG(); 0;})
 #define erratum_set_next_event_tval_phys(...)          ({BUG(); 0;})
 #define erratum_handler(fn, r, ...)                    ({false;})
+#define arch_timer_this_cpu_has_cntvct_wa()            ({false;})
 #endif /* CONFIG_ARM_ARCH_TIMER_OOL_WORKAROUND */
 
 static __always_inline irqreturn_t timer_handler(const int access,
@@ -674,8 +694,15 @@ static void arch_counter_set_user_access(void)
                        | ARCH_TIMER_VIRT_EVT_EN
                        | ARCH_TIMER_USR_PCT_ACCESS_EN);
 
-       /* Enable user access to the virtual counter */
-       cntkctl |= ARCH_TIMER_USR_VCT_ACCESS_EN;
+       /*
+        * Enable user access to the virtual counter if it doesn't
+        * need to be workaround. The vdso may have been already
+        * disabled though.
+        */
+       if (arch_timer_this_cpu_has_cntvct_wa())
+               pr_info("CPU%d: Trapping CNTVCT access\n", smp_processor_id());
+       else
+               cntkctl |= ARCH_TIMER_USR_VCT_ACCESS_EN;
 
        arch_timer_set_cntkctl(cntkctl);
 }
@@ -798,16 +825,7 @@ static void __init arch_counter_register(unsigned type)
                else
                        arch_timer_read_counter = arch_counter_get_cntpct;
 
-               clocksource_counter.archdata.vdso_direct = true;
-
-#ifdef CONFIG_ARM_ARCH_TIMER_OOL_WORKAROUND
-               /*
-                * Don't use the vdso fastpath if errata require using
-                * the out-of-line counter accessor.
-                */
-               if (static_branch_unlikely(&arch_timer_read_ool_enabled))
-                       clocksource_counter.archdata.vdso_direct = false;
-#endif
+               clocksource_counter.archdata.vdso_direct = vdso_default;
        } else {
                arch_timer_read_counter = arch_counter_get_cntvct_mem;
        }
-- 
2.11.0